Bone marrow transplantation in lethally irradiated mice was used to investigate immunogenetic factors affecting the severity of the graft-versus-host reaction. A high leukemic strain of mice was treated with lethal irradiation and inoculated compatible marrow to study incidence, latency and genotype of the leukemias. Leukemia induction in donor marrow occurred earlier when the marrow was most compatible with the recipient. Two methods of altering the incidence of leukemia were investigated. The most effective method apparently alters the environment while the other apparently alters the susceptibility of the transplanted cells to malignant transformation. BIBLIOGRAPHIC REFERENCE: Uphoff, D.E.: Mechanisms of graft-versus-host reaction: An immunogenetic appraisal. Transplantation Proc. 8: 515-519, 1976.